Lakes & Ponds Still Waters brimming with life
Lakes & Ponds Objectives
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Classify lakes based on productivity and identify three different lake zones.
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Write the general process of photosynthesis and provide example of photosynthetic pond organisms.
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Provide examples of pond consumers, both microscopic and macroscopic, and explain the cascade hypothesis using a lake food web.
Lakes contain water that primarily stays within a basin. There can be water movement on the surface or some water entering and leaving, but the bulk of water is not moving from one point to another (like in a stream).
Ponds are small lakes that can be strongly impacted by seasonal changes, sometimes completely freezing or drying out.
Lakes are classified based on productivity, the amount of photosynthesis occurring in lake producers.
Oligotrophic lakes have low levels of productivity, these are often clearer water.
Mesotrophic lakes have a middle level of productivity, enough photosynthesis to support a more complex food web.
Eutrophic lakes are high in productivity, so high they can be green in color.
Crater Lake is an example of an oligotrophic lake. Productivity is low and the water is clear to well over a meter below the water’s surface.
Within a single lake, there are different zones that are impacted by the amount of available light and nutrients. Each zone can have its own organisms.
The Littoral lake zone is the area near shore that is shallow enough for light penetration. Many amphibians and aquatic insects spend part of their life cycle here.
The Limnetic lake zone is the open water area that sunlight penetrates. Many large fish swim here.
The Benthic lake zone is the bottom, below the sunlit limnetic zone. Although light is low, nutrients are high and scavengers like crayfish and catfish can be found here.
Some species utilize different zones in a lake during different life stages. For example, fish eggs may be laid on plants in the near shore littoral zone, but later stages of larger fish may swim and eat in the open limnetic zone.
Eutrophic lakes and small ponds are typically very productive, meaning there are many producers, including algae and plants. Producers carry out the process of photosynthesis.
Photosynthesis is the combination of carbon dioxide and water in the presence of sunlight energy, producing oxygen and energy-rich sugars. Only producers carry out this process.
Respiration is the opposite reaction: using sugars and oxygen and producing carbon dioxide, water, and energy. All organisms carry out some form of this process.
These are common microscopic producers that you are likely to find in a lake or pond.
Cyanobacteria
Algae
Diatoms
Euglena
These are producers; plants with different strategies for obtaining sunlight.
Elodea
Azolla
Water Lily
Duckweed
Duckweed and Azolla are not rooted into the soil like water lily plants; they float on top of the water. From earlier this term, what organims live within the Azolla, providing nitrogen?
Azolla
Aquatic plants found in local ponds have strategies for continuing photosynthesis in conditions that could impact gas exchange or light availability.
The leaves of this water lily plant have stomata located on the top of the leaves instead of on the bottom as you observed in an earlier lab. Why?
All three of the species below are submerged aquatic plants. Water is abundant, but what two other components of productivity can be limited?
Answer: light & nutrients
Elodea
Hornwort
Vallisnaria
Here are a few examples of common microscopic lake and pond consumers.
Paramecium
Amoeba
Rotifers
Copepods
These are a few commonly observed macroscopic (but still small) consumers.
Daphnia
Hydra
Nematodes
Planaria
A food web would not be complete without decomposers. In aquatic ecosystems, bacteria are the primary decomposers.
In the middle of this photo is a single photosynthetic diatom. The smaller brownish material around it may be bacteria, they are more difficult to identify.
Sunlight energy continually enters a pond wood web, and is converted by photosynthetic producers into energy-rich sugars. Organisms convert that sugar energy into heat that is lost from the system.
Decomposers enable nutrients from dead organisms to re-enter the food web’s producers, cycling repeatedly over time.
Within ecosystems, an impact on one trophic (feeding) level can cascade (move) through other trophic levels. For example, if you add large fish to a lake, it can impact the amout of phytoplankton even if the large fish do not directly eat the phytoplankton. Looking at the simplified food chain, can you explain why?
Answer: an increase of large fish would decrease the small fish and insects they eat. Without those small fish and insects, the organisms they eat (smaller consumers) would increase. They eat phytoplankton, so an increase in the herbivores would decrease the amount of producers like algae.
Start this Guide’s first journal page here
Journal Page #9: Lake Food Web
Sketch and label a hypothetical lake food web. Include the following in your labeled sketch:
- three lake zones (littoral, limnetic, benthic)
- examples of producers (microscopic and macroscopic)
- examples of consumers (microscopic and macroscopic)
- decomposers
- arrows for energy flow (including sun and heat)
- arrows for nutrient cycling
You are turning in a journal page of your handmade (either on paper or digitally produced) lake food web that contains all of the required sketched and labeled components.
It may help to review food webs here.
The next section explores wetlands that are partially aquatic and partially terrestrial.
Check your knowledge. Can you:
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classify lakes based on productivity and identify three different lake zones?
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write the general process of photosynthesis and provide example of photosynthetic pond organisms?
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provide examples of pond consumers, both microscopic and macroscopic, and explain the cascade hypothesis using a lake food web?
Go back to the Freshwater Guide Page
Go forward to the Wetlands Page
Freshwater Guide Contents
Complete all four of these sections before taking the quiz and submitting your two journal pages.
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